Influence of the Dew Point on Coating Quality of GI/GA TRIP steels

Abstract

The selective oxidation and the reactive wetting of CMnSi Transformation-Induced Plasticity steels during Galvanizing and Galvannealing were investigated by FE-EPMA and high resolution Transmission Electron Microscopy. During intercritical annealing in a N2+10%H2 gas atmosphere with a dew point in the range of -60˚C to +5 ˚C, a continuous layer of selective oxides was formed on the surface. In a low dew point atmosphere, Mn-rich oxide selectively formed on the surface during heating to 820˚C. Film type of Si oxides and manganese-silicon compound oxides covering the surface were formed during intercritical annealing at 820˚C. In the atmosphere with a dew point of -10˚C, the transition of external to internal oxidation was observed. Regions of external and internal oxidation developed separately. In the high dew point atmospheres, both external and internal oxidation occurred leading to the formation of mixed manganese-silicon oxides started during the heating to 820˚C. The process progressed during intercritical annealing at 820˚C. Simultaneously, small sub-surface ferrite grains formed which had a grain boundary network of manganese-silicon oxides. Annealing in high dew point gas atmosphere resulted in a thinner layer of external oxidation and a greater depth of Mn and Si internal oxidation. The galvanizing simulation was carried out in a Zn bath containing 0.22 mass% Al and the bath temperature was 460 °C. Coarse and discontinuous Fe2Al5-xZnx grains and Fe-Zn intermetallics (ζ and δ) were observed at the steel/coating interface after the hot dip galvanizing of panels annealed in a low dew point atmosphere (-60 ˚C). Poor wetting was observed on panels annealed in a low dew point atmosphere due to the formation of thick film-type oxides on the surface. After annealing in a higher dew point gas atmosphere (-10 °C, and +5 °C), the Fe2Al5-xZnx was continuous and fine grained. No Fe-Zn intermetallics were formed. The small grain size of the inhibition layer was attributed to the nucleation of the Fe2Al5-xZnx grains on fine ferrite subsurface grains and the presence of granular surface oxides. The galvannealing simulation were carried out in a Zn bath containing 0.13 mass% Al and the bath temperature was 460 °C. The galvannealing temperature was in the range of 540-570 ˚C. After the hot dip galvannealing of a panel annealed in a low dew point atmosphere (-60 ˚C), the coating layer was consisted of delta and eta phase crystals. Al and the oxides formed during annealing were present mostly at the steel/coating interface. After the hot dip galvannealing of a panel annealed in higher dew point atmospheres (-10 ˚C and +5 ˚C), the coating layer mainly consisted of delta phase crystals, and thin layer of gamma phase crystals were present at the steel/coating interface. Two types of delta phase crystals were observed: the coarse delta and the fine delta phase crystals. Al was detected mainly at the surface of the coating layer. Oxides were distributed all around the coating layer. The reduction of the surface oxides layer thickness on panels annealed in a high DP gas atmosphere resulted in increased Fe content at the surface of the coating layer. The improved coating quality of the panels annealed in higher dew point gas atmospheres was due to the formation of thinner layer of oxides. A high dew point atmosphere can therefore significantly contribute to the decrease of Zn-coating defects on CMnSi TRIP steels processed in hot dip galvanizing/galvannealing lines.