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Title: Cleanness and castability effects on the surface quality of formable steels
Author: Way, L. D.
Awarding Body: University of Wales Swansea
Current Institution: Swansea University
Date of Award: 2003
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Increased demand for high quality formable sheet steel for exposed applications has led to global adoption of the formable interstitial free (IF) steels. However, these steels are susceptible to unsightly surface line defects. The aim of the present study was to improve current understanding of the origins of typical line defects in IF steels so suitable cost-effective countermeasures can be derived. Uncoated, cold rolled steel were found to contain aluminium/complex oxide based line defects. These features were shown to originate by entrapment on non-metallic inclusions (NMIs) in steel slabs during continuous casting. Progressive deposition of alumina NMIs on casting refractors was a significant contributing factor. Characterisation of alumina NMIs formed across a range of full scale steel manufacturing conditions highlighted a number of areas of improvement. Iron oxide defects containing varying degrees of internal oxidation were found in galvannealed NbTi-IF steel These 'FexOy slivers' organised from Ar bubbles <2mm in diameter, trapped in slabs during continuous casting operations. During slab reheating, any such features exposed at the steel surface will allow significant internal oxidation of the underlying steel. Subsequent rolling operations cause the oxidised region to elongate, thus producing the final sliver defect. Elimination of this feature thus requires fewer Ar bubbles present in shallow subsurface locations in the cast slab. Short term quality improvements can be achieved via continuous improvement of operating practices. However, any significant reduction in defect incidence requires a more radical approach. One possible solution is thus presented by exploiting classical 'clean steel' theory.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (D.Eng.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available