Oxidation phenomena in advanced high strength steels : Modelling and experiment

Abstract

Galvanized advanced high strength steels (AHSS) will be the most competitive structural material for automotive applications in the next decade. Oxidation of AHSS during the recrystalization annealing process in a continuous galvanizing line to a large extent influences the quality of zinc coating on the final galvanized steel product. For example, formation of oxides of alloying elements (e.g. Mn, Cr, Si) at the steel surface during annealing prior to galvanizing leads to poor adhesion of a zinc coating. Yet, knowledge on the high temperature oxidation behaviour of AHSS is rather limited. The primary aim of this thesis is to provide fundamental understanding on the kinetics of internal oxidation of AHSS during annealing. The classical Wagner internal oxidation theory for binary alloys was extended to account for multi-component alloys. To this end, a generic coupled thermodynamic-kinetic internal oxidation model based on Fick’s 1st law was developed in order to predict the kinetics of internal oxidation, as well as the concentration depth profiles of internal oxides and solute elements in alloy matrix, considering the finite solubility product of oxide precipitates, the non-ideal behaviour of solid solution and the formation of multiple type of oxide species. The internal oxidation behaviour of Fe-Mn and Fe-Mn-Cr steel alloys were experimentally studied to validate the model. It has been found that for Fe-Mn and Fe-Mn-Cr steel alloys, the effect of non-ideal behaviour of solution on internal oxidation is negligible, and local thermodynamic equilibrium is established within internal oxidation zone. Besides, the kinetics of Wustite formation on pure iron and Mn alloyed steels annealed in CO2 + CO or H2O + H2 gas mixtures as well as the reduction kinetics of the Wustite scale in Ar + H2 gas mixtures were investigated. The growth of Wustite scale on iron and Mn alloyed steels follows the linear rate law. However, adding Mn to iron, even at a relatively low concentration (say 1.7 wt%), dramatically lowers the growth rate of Wustite scale. Nevertheless, reduction kinetics of the Wustite scale on iron and Mn alloyed steels are almost the same. During the reduction process a dense iron layer is formed which separates the remaining Wustite scale from the reduction atmosphere. The rate of Wustite reduction by H2 is controlled by the diffusion of solute oxygen dissolved in the formed iron layer.