Effect of Dew Point on the Selective Oxidation and Reactive Wetting of DP980 Steel during Hot-Dip Galvanizing

The hot-dip galvanizing behavior of an advanced high-strength steel DP980 is investigated, focusing on selective oxidation and reactive wetting, as compared with interstitial-free (IF) steel. The steels are annealed with varying dew points of -50, -25, and 0 degrees C, and then immersed in a galvannealing bath containing 0.13 wt% dissolved Al. The results show that higher concentrations of Mn and Si elements in DP980 steel result in an increase of nonwettable Mn2SiO4/MnSiO3. The increase in dew point does not promote the internal oxidation of alloying elements in DP980 as in IF steel, and DP980 displays the lowest external oxidation at -50 degrees C. After galvanizing, Mn and Si oxides are observed above the inhibition layer, which is due to the oxide cracking caused by the mismatch of thermal expansion coefficient between the surface oxides and the steel, and then the oxide flaking and incorporation are caused by the interfacial infiltration through cracks and the formation of intermetallic crystals of Fe-Al and subsequent Fe-Zn. Therefore, IF steel maintains good wetting in the presence of external oxides, while DP980 steel exhibits more external film oxides and poor coating quality at high dew points. The selective oxidation of alloying elements in an advanced high-strength steel DP980 is investigated, and compared with IF steel, DP980 steel shows more serious surface oxidation and poor coating quality at high dew point. After galvanizing in zinc bath containing 0.13% Al, flaking of oxides is occurred as well.image (c) 2024 WILEY-VCH GmbH