12th International Conference on Zinc & Zinc Alloy Coated Steel Sheet - GALVATECH 2021

Under certain conditions of oxygen shortage and high temperatures as could occur after coiling a hot-rolled strip, alloying elements with high oxygen affinity can diffuse and oxidise preferably along the grain boundaries. This selective internal oxidation can be increased by additional heat induced from phase transformations in the coiled state and depends on steel composition. This diffusion to the grain boundaries and subsequent oxidation causes a depletion of solute alloying elements in a surface near area.

When oxides are present along grain boundaries, pickling can alter the surface topography by selective dissolution of these oxides on the µm-scale (µ-topography). This altered topography and the depletion of alloying elements influence diffusion properties during intercritical annealing prior hot dip galvanizing, leading to changed surface segregation of alloying elements.

This effect was exemplified by pickling and subsequently cold rolling a hot rolled high strength steel strip with and without internally oxidized grain boundaries, using an adapted process. Samples with and without altered µ-topography were galvanized in a hot-dip simulator with varying annealing parameters. The annealed and galvanized samples where characterized in terms of surface enrichment of alloying elements by GDOES, inhibition layer density by potentiostatic dissolution and SEM measurements, as well as crash relevant automotive zinc adhesion tests.

Surfaces with altered µ-topography show higher reactivity through lower surface segregation of alloying elements and can build more dense inhibition layers during hot-dip galvanising under identical annealing conditions. This effect could be helpful to enhance galvanisability of future difficult steel grades.