Surface Conditioning of High Strength Steels by Gas-Metal Reactions Prior to Hot-Dip Galvanising

摘要

Annealing in reactive atmospheres has shown to be an additional tool for modifying the chemical composition of the steel surface and thus improving the wetting conditions and the hot-dip coating behaviour of the steel DP500. Using CO-H_2-N_2 atmospheres the effects are: CO as a carbon donator allows in conjunction with H2 very fast carburising kinetics The mechanical properties are affected when applying carbon activities a_C > 1. Carburising in CO-H_2-N_2 atmospheres does not reduce the enrichment of alloying elements such as Si, Mn and Cr at the surface. The CO-H_2-N_2 gas atmosphere offers oxygen at the surface due to the reaction CO ->[C] + O_(ad). The increased amount of O_(ad) provokes the formation of oxides of the elements Si, Mn and Cr. By applying carburising activities of > 1 the wettability of the steel surface during immersion into the zinc melt is deteriorated and the formation of the inhibition layer is disturbed. An inhomogeneous coating results. To minimise the oxygen offer at the surface, the amount of H_2 should be increased or carburisation should be carried out employing other carbon donators such as CH_4.