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Surface conditioning of modern high-strength steels prior to hot-dip galvanising was considered as a way to improve the hot-dip coating quality by influencing the enrichment of the alloying elements at the steel surface. As one possible approach, Fe flashes were examined in detail. There was a texture effect present and Si diffusion could not be hindered by an Fe layer of 1 µm thickness. Carburising altered the formation and composition of the inhibiting layer. It became inhomogeneous and the wettability of the substrate subsequently deteriorated. Due to an increased amount of oxygen produced by the dissolution of CO, carburising enriched Si, Mn and Cr below the surface. On the one hand, nitriding improved the coating quality but, on the other hand, increased the yield and tensile strength while elongation decreased. The adhesion of the coating was generally good, but samples treated with medium and higher NH3 content showed cracks within the steel. To conclude, nitriding is the more promising reactive annealing treatment tool for improvement of the hot-dip coating behaviour of high-strength steels. The complete or partial removal of the oxide skin by using mechanical abrasion, arc treatment and laser application is expected to result in an improvement of the subsequent galvanising process. The effectiveness of reducing enrichments by various methods has been demonstrated. Although the surface layer removal by laser treatment was effectively in terms of removing residual oxide layers, bare spots have been observed in the laser-treated area. A possible relationship between topographic structuring introduced energy per area and the removal result was investigated. In addition, the Fe2AI5 intermediate layer formation induced by pre-treatment of the sheets with a pulsed beam has been investigated.

Additional information

Authors: SCHUMACHER B, Thyssen Krupp Stahl AG, Duisburg (DE);LEUSCHNER R, Thyssen Krupp Stahl AG, Duisburg (DE);BESTE D, RWTH, Aachen (DE);CLAESSENS S, OCAS, Zelzate (BE)
Bibliographic Reference: EUR 21421 EN (2005), 154 pp. Euro: 25
Availability: Katalogue Number: KI-NA-21421-EN-S The paper version can be ordered online and the PDF version downloaded at:
ISBN: ISBN: 92-894-9904-4
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