Development of the surface structure of TRIP steels prior to hot-dip galvanizing
Introduction
Transformation-induced plasticity assisted steels (TRIP steels) are ideal candidates for lightweight automotive applications due to their high strength to weight ratio, allowing thinner cross-sections to be used for a given application [1]. Furthermore, the good uniform elongation and high work hardening rate of TRIP steels provide high strength without compromising ductility, formability and vehicle crashworthiness [1]. However, the use of thinner cross-sections results in corrosion protection becoming increasingly important in order to maintain structural integrity and satisfy consumer durability expectations. The continuous galvanizing process is among the most cost-effective means of achieving this objective, but when using conventional hot-dip galvanizing process parameters there can be problems with poor reactive wetting between the liquid zinc and the surface of TRIP steels [2], [3], which is necessary for the formation of the interfacial inhibition layer (Fe2Al5) and is integral to producing coatings with high ductility [4]. Poor wetting is usually caused by the selective oxidation of the Mn, Si and Al that are commonly used as alloying elements in these steels, i.e., they form oxides that are not reduced by conventional atmospheres employed in continuous galvanizing.
TRIP steels have conventionally contained C, Mn and Si [5], where C and Mn are used to stabilize austenite and Si is used as a solid solution strengthener and to suppress carbide formation [6], [7]. Silicon often causes problems during hot rolling, producing an oxide scale that is difficult to remove in subsequent processing [8], [9]. Silicon has also been shown to be problematic during hot-dip galvanizing [2], causing poor wettability as a result of a layer of amorphous SiO2 present on the surface [10]. It has also been shown that Mn and Si can form the mixed oxide Mn2SiO4, which was also found to be detrimental to the wettability of the steel [11]. Due to the adverse affects of Si during continuous galvanizing, Al has been selectively substituted for Si in TRIP steels and has been shown to improve reactive wetting during galvanizing [11]. Al has also been shown to have a similar effect to Si on TRIP steel microstructural evolution, preventing carbide precipitation and providing a sufficient amount of retained austenite that is stable at room temperature [12]. De Meyer et al. [8] have shown that Al can partially replace Si to produce TRIP steels with comparable strength and superior ductility to those with a higher Si content. In this study, TRIP steels alloyed with C, Mn and with Al either completely or partially replacing Si have been used for the purpose of investigating the reactive wetting of TRIP steels during continuous galvanizing.
In this investigation, the galvanizing of two TRIP steels and one C–Mn steel was studied. The wettability of the alloys by liquid Zn(Al, Fe) was investigated (i) by the visual appearance of the galvanized panels and (ii) by observing the inhibition layer (Fe2Al5) at the Fe–Zn interface. The wettability of the steel will be related to the surface structure prior to galvanizing, focusing on the selective oxidation of alloying elements at the surface. The depth to which the oxides penetrate, the chemical composition of the oxides and the distribution of oxides on the surface was studied. The steel surface morphology was also observed at various times during the annealing cycle to determine how the surface structure evolved with annealing time.
Section snippets
Experimental method
The chemical compositions of the experimental steels are shown in Table 1. The TRIP steels were fabricated at the CANMET Materials Technology Laboratory and the C–Mn steel was supplied by Stelco Inc.
Experimental heat treatment consisted of heating the steel to the intercritical annealing temperature at which the microstructure would be 50% α–50% γ. This temperature was determined using Thermocalc® to be 862 °C for the 1.5% Al TRIP steel and 825 °C for the 1% Al TRIP steel. The C–Mn steel was
Surface structure prior to galvanizing
Using XPS, the oxidation states of Fe, Mn, Si and Al can be determined and the surface species identified where information on their binding energy was available [17], [18], [19], [20], [21], [22], [23], [24]. Measured binding energies taken from the XPS spectra for the C–Mn 825 °C steel, 1.5% Al TRIP steel and the 1% Al TRIP steel for both annealing atmospheres after the ‘460 °C Hold’ are summarized in Table 3. The results for the C–Mn 862 °C samples did not differ significantly from those of the
Dissolution of Mn from the steel strip in the Zn bath
From the surface quality of the galvanized panels and the fully developed inhibition layer, it was determined that the C–Mn steel panels demonstrated good wettability at both of the dew points and intercritical annealing temperatures that were investigated. From these results it can be concluded that an alloy content of 1.5 wt.% Mn does not a have a detrimental effect on the reactive wetting of this steel despite the presence of significant quantities of manganese oxide at the surface prior to
Conclusions
The galvanizing behavior of a C–Mn steel and two TRIP steels was investigated, focusing on the role of selective oxidation of the alloying elements on reactive wetting of the steel surface. From the experimental results the following conclusions were developed:
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Mn and Si showed a strong tendency towards external oxidation under both annealing atmospheres and all alloys.
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The results show a tendency towards the internal oxidation of Al, with a greater degree of internal oxidation being shown for
Acknowledgements
The authors would like to thank Stelco Inc., the Natural Sciences and Engineering Research Council of Canada (NSERC) and the members of the McMaster Steel Research Centre for support of this research through their funding of the Stelco/NSERC Industrial Research Chair in Steel Product Application. The authors would also like to thank Mr. Jason Lavallée (galvanizing simulator), Mr. John Rodda, Mr. Doug Culley, Mr. Chris Butcher and Mr. Steve Koprich for their technical support and Dr. Li Sun
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