Elimination of Wettability Issues in Interstitial Free High Strength (IFHS) Steel by Controlling the Annealing Atmosphere

In this investigation, a thermodynamic study was performed to predict the type and amount of possible oxides formed on interstitial-free high-strength (IFHS) steel substrate surface during the annealing treatment prior to dipping in a liquid zinc alloy bath. This study was carried out based on the CALPHAD approach using the thermodynamic simulation software package Thermo-Calc by varying the annealing temperature and annealing atmosphere (dew point and gas composition). The experiments were performed on IFHS-350 grade steel substrate using hot-dip process simulator (HDPS) for varying annealing conditions (temperature: 780-830 degrees C and dew point: - 30 to + 10 degrees C). The specimens were characterized in as-annealed condition as well as after hot dipping in the galvanizing bath. This investigation was performed to find out the suitable combinations of annealing temperature and dew point for developing a good quality coating on IFHS grade steel substrate by eliminating the wettability issues. Good quality coating, free from bare spots, was obtained using an annealing gas atmosphere having 5%H2 and 95%N2 at (a) 780 degrees C with - 30 degrees C dew point, (b) 805 degrees C with - 10 degrees C dew point and (c) 830 degrees C with + 10 degrees C dew point. In general, coating quality was improved with low annealing temperatures and low dew point or high annealing temperature and high dew point conditions.