Abstract
A high-strength strip steel (yield strength of ~700 MPa) with a ductile-brittle transition over a wide temperature range shows splits (fissures) on the fracture surfaces in the upper transition region. The steel, hot rolled to strip thicknesses of approximately 10.0 mm and 16.8 mm, had a predominantly fine-grained ferrite microstructure with some coarse grain patches (area percent of 11 pct and 42 pct, respectively). Low-blow Charpy tests were carried out at room temperature, corresponding to the upper transition region for these strips. The low-blow tests resulted in the formation of splits without main crack propagation from the notch; therefore, the energies at which the splits initiated could be determined. Acoustic emission (AE) sensors were used during low-blow Charpy testing of the strip steels and mild steel (where no splits occur); it was found that AE was able to detect signals from the hammer impact, split formation, and ductile deformation. Optical microscopy, scanning electron microscopy (SEM), and X-ray tomography were carried out, which verified the presence of splits and showed that they propagated by transgranular cleavage, preferentially following coarse-grained regions. No significant difference in the number or length of splits between the 10- and 16.8-mm strip was observed, but the 10-mm strip did produce deeper splits during the low-blow Charpy testing. The deeper splits contribute to a lower impact transition temperature for the 10-mm strip material.
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Acknowledgments
The authors would like to thank the School of Metallurgy and Materials at the University of Birmingham for the provision of research facilities, and Tata Steel U.K. Limited for provision of test material, data, and financial support of the project along with EPSRC for financial support. Thanks are due to Carl Slater for assistance with Matlab reconstructions, to Andrey Kostryzhev for the AE testing, to Michelle Holder and the School of Dentistry for the use and assistance of the Micro CT, and to Dave Crowther of Tata Steel for helpful discussions.
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Manuscript submitted December 9, 2011.
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Punch, R., Strangwood, M. & Davis, C. Origin and Propagation of Splits in High-Strength Low-Alloy Strip Steel. Metall Mater Trans A 43, 4622–4632 (2012). https://doi.org/10.1007/s11661-012-1307-1
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DOI: https://doi.org/10.1007/s11661-012-1307-1