Wetting force and contact angle measurements to evaluate the influence of zinc bath metallurgy on the galvanizability of high-manganese-alloyed steel
Introduction
High-Mn-alloyed austenitic steel grades with 15.0–30.0 wt % Mn, e.g. commercial L-IP®- [1] or X-IP®-steel [2], [3] concepts, offer high strength in combination with high ductility due to mechanical twinning as the dominating deformation mechanism [4] (so-called TWIP-effect, from twinning induced plasticity). There exists high potential for the use of high-Mn-alloyed steels in structural sheet parts for automotive body applications to reduce weight as well as increase passenger safety. But because of their alloying concept, high-Mn-alloyed steels require an anti-corrosion coating. Unfortunately, cost-effective hot-dip galvanizing fails. Wetting by the zinc melt is inadequate due to distinctive selective oxidation of alloying elements, e.g. Mn, during recrystallization annealing treatment prior to hot-dipping [5], [6], [7].
However, not only annealing, but also zinc bath conditions are known to contribute to the wetting result. Industrial zinc baths used to continuously hot-dip galvanize steel strips are typically alloyed with about 0.12–0.25 wt % Al to the zinc balance and Fe saturated. This Al-addition leads to the desired formation of a Fe2Al5 interface layer between steel and coating improving adhesion of the zinc coating and inhibiting generation of brittle Fe-Zn-intermetallics [8], [9], [10], [11], [36], [37]. Referring to this, the present work aims to evaluate the influence of zinc bath metallurgy on wettability of a high-Mn-alloyed steel concept and support further process developments in hot-dip galvanizing of (high-)Mn-alloyed steel concepts. This evaluation is to be carried out by means of wetting force as well as contact angle measurements. Wetting force and contact angle measurements using sessile-drop [10], [12], [13], [14] or Wilhelmy plate methodology [7], [11], [15] are established experimental approaches to investigate zinc wettability on steels. Compared to mainly used hot-dip galvanizing simulation, these methods offer the advantage of obtaining a quantitative wetting result with reduced experimental effort. In the framework of the present investigation, variation of zinc bath metallurgy was focused on two issues:
- 1)
Variation of the Al content to optimize zinc bath composition for hot-dip simulation on the one hand. Otherwise reduction potential of the bath-Al on external MnO at the steel surface should be proof. An aluminothermic MnO reduction in-situ to reactive zinc wetting is currently discussed to explain additional formation of FeZn13-(ζ)-phase by hot-dip galvanizing of Mn-alloyed steel [15], [16], [17], [18], [19], [20], [21].
- 2)
Adding Si and Mn to the zinc bath to modify wetting reaction. Si is known to create a Fe-Al-Si-interface layer steel/coating [10], [22], [23]. In contrast to that, adding Mn to the zinc bath is known to improve formation of Fe-Zn-intermetallics [9], [24]. Thus, improved wettability might be potentially achieved by such a modified wetting reaction.
Hence, the present work constitutes a basic investigation to achieve knowledge and understanding about trends in wetting as well as wetting reaction of high-Mn-alloyed steels. Detailed information about the influence of zinc bath metallurgy on mechanical or electrochemical properties of a hot-dip galvanized coating requires hot-dip simulation trials or plant try-outs, which could be based on the findings of the present work.
Section snippets
Materials and methods
Samples measuring 45.0 × 15.0 × 1.0 mm were prepared by laser-cutting out of an industrially produced and cold-rolled Fe-Mn-C-steel sheet containing 23.0 wt % Mn, commercial grade X-IP®1000 (Table 1), and ultrasonically cleaned.
The present investigation was performed using the Wilhelmy plate methodology. This methodology is often used to measure the surface tension of a liquid γl by measuring the change in weight ΔFG of a defined plate in contact with the liquid. The value of ΔFG equates to the
Variation of Al-content
Wetting result increases with increasing wetting force FW and decreasing contact angle θ. The measurement results of FW and θ concerning the variation of bath-Al (baths 1–9) are illustrated by Fig. 4a–d. Regarding these measurements the optimum of wetting result occurred if about 0.10–0.25 wt % Al was added presenting the typical Al-level in industrial galvanizing lines [33], [34]. Bath 3 containing 0.18 wt % Al achieved the best wetting result of this experimental series (Fig. 4a, b). Addition
Conclusion
Wetting force and contact angle measurements were performed to investigate the influence of zinc bath metallurgy in terms of Al-content as well as Si- and Mn-additions on galvanizability of high-Mn-alloyed steel concepts. Therefore, a commercial Fe-Mn-C steel concept containing an alloying amount of about 23.0 wt % Mn was used and heat treated in-line under close-to-production annealing conditions. The following conclusions can be drawn:
- −
Wetting force and contact angle measurements using the
Acknowledgement
The authors would like to thank ThyssenKrupp Steel Europe AG for providing the investigated material and also gratefully acknowledge the support of the colleagues of the Centre of Materials Excellence of ThyssenKrupp Steel Europe AG as well as Dortmunder OberflächenCentrum GmbH in terms of analytical work and discussion.
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