Galvanizability of high-strength steels for automotive applications
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duate Student, and B.C. De COOMAN, Professor, are with the Laboratory for Iron and Steelmaking, Gent University, B-9052 Zwignaarde (Gent), Belgium. S. CLAESSENS is with OCAS NV, Research Centre of the SIDMAR Group, ARBED Group Flat Rolled Products Division, B-9060 Zelzate, Belgium. Manuscript submitted April 6, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
combined with scanning electron microscopy (SEM) observations were made to characterize the steel surface prior to dipping and to determine the origin of the problem. It was found that the steel surface had a large amount of islandlike features, surrounded by continuous boundaries with a dark backscattered electron (BSE) contrast (Figure 1, left). Energy dispersive x-ray (EDX) measurements revealed that these boundaries were enriched with Si up to 5 wt pct. An explanation for this observation may be the partitioning of the Si between the austenite grains and the ferrite grains during intercritical annealing. The higher diffusion coefficient of Si in austenite[3] and the high solubility of Si in ferrite cause the Si to diffuse from the austenite toward the ferrite, where it becomes enriched at the grain boundary and is readily oxidized by the O in the atmosphere. The Si oxides thus formed give the dark BSE contrast. It has been known for some time that increasing the dew point of the atmosphere during annealing results in a change from external to internal oxidation.[4,5] Therefore, additional tests were done at a dew point setting of ⫹10 ⬚C up to the start of the intercritical annealing, followed by a decrease to ⫺50 ⬚C during the remainder of the cycle to avoid external oxidation during the IBT. The SEM images of the surface of the steel annealed in this high dew point atmosphere revealed no islandlike features, as in case of the low dew point setting, but very small particles, scattered over the surface (right part of Figure 1). The high O2 content in the atmosphere enables the transition from external oxidation to internal oxidation, leaving fewer oxidized phases on the steel surface.[4] This was confirmed by XPS measurements, which showed that Mn and Si were present at the surface in high concentrations, but also that a pronounced decrease in Si and Mn was obtained by increasing the dew point (Figure 2). This is in agreement with Grabke et al.[6] and Bordignon et al.,[7] who mention the segregation of these elements to the surface and their effect on the wettability by liquid Zn in ultralow carbon steels, alloyed with Si, Ti, and/or Nb. After hot dip galvanizing experiments, the coated panels were examined visually to evaluate their galvanizability. It was found that in the low dew point atmosphere, common to most hot dipping, severe problems of wettability were encountered. The steel surface remained mostly bare and covered with a number of isolated Zn drops (Figure 3). The wetting by the Zn considerably improved if a higher dew point atmosphere was used for the annealing. Nevertheless, a poor quality Zn coating with many bare areas was obtained. A transmi
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