Understanding the Mechanisms of Fe Dissolution from the Steel Strip into the 55%Al-Zn Coating Alloy Bath
Creators: name: Islam, Syed
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 4017
subject: Mechanical engineering
subject: FOS: Mechanical engineering

description: In the continuous hot-dip metal coating process, when the steel strip enters the bath, iron (Fe) dissolves from the strip into the molten coating alloy bath. A portion of the dissolved Fe takes part in the formation of an alloy layer on the steel strip while the remaining Fe forms undesirable intermetallic compounds (IMCs), also known as dross in the coating alloy bath. The dissolution of Fe and dross formation are a long-standing issue in the metal coating industry. Excessive dross formation significantly affects the coating process and the final product quality. This thesis presents a systematic experimental investigation of the influence of the major process parameters, substrate preheat temperature and strip residence time on Fe dissolution in the hot-dip coating process. The primary objective is to understand the mechanism(s) of Fe dissolution from the steel strip into the coating alloy bath during the continuous hot-dip metal coating process. To tackle the Fe dissolution issue, a novel experimental technique, based on the Fe-foil dipping approach, has been developed. The approach is based on dipping Fe-foil samples into a coating alloy bath at 600oC using a hot dip process simulator (HDPS). The dipping time was fixed at 1 second (s), which is the shortest possible dipping time allowed by the simulator. An innovative panel sandwich device is designed for dipping the Fe-foils into the coating alloy. Fe-foil samples of thickness of 5 µm, 10 µm and 20 µm were dipped at different preheat temperatures from 500oC to 605oC. This approach enabled a systematic quantitative study of the dissolution of Fe into the 53%Al-43.4%Zn-1.6%Si-2%Mg (AM) alloy coating bath. In contrast, it is difficult to quantify a small variation (&lt;0.5µm) in the thickness of a commercial strip sample, where the thickness of the steel strip is in the range of 0.4 to 1.6 mm. This could be a major reason why all previous efforts were unable to provide direct evidence of the dissolution of Fe from the strip surface in the coating pot. Metallic coated samples, both industrially coated steel strips and those coated in laboratories, were extracted and characterised using scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS) and the focused ion beam (FIB) milling techniques. Thermodynamic simulation is performed to understand the metastable solubility of Fe at the steel/coating bath interface with respect to the industrial production condition using the Pandat database and software. The experimental results show a clear reduction in Fe-foil thickness with respect to preheat temperature, confirming direct dissolution of Fe from the Fe-foil into the AM alloy coating bath. In addition, there was a deflection temperature (540oC) above which the Fe dissolution amount shows a linear dependency on entry temperature, while below it is entry-temperature independent. This turning point corresponds to the approximate temperature for the crystallisation of α-Al from the AM alloy on the Fe-foil. Theoretical calculations revealed that the driving force for the dissolution of Fe is significant in terms of the difference between the chemical potential of Fe on the surface of the Fe-foil (100 wt.% Fe) and that of the Fe in the AM coating bath (0.41 wt.% Fe). This substantial driving force together with the high mobility of Fe on the oxide-free steel surface causes a rapid dissolution of Fe into the AM coating alloy bath. More specifically, the dissolution process can be understood as follows: although the bath is saturated with Fe (0.41�) in equilibrium with the τ5c IMC particles, it will immediately become under-saturated with the Fe-foil or steel strip surface (~100 wt.�) at the interface when the Fe-foil or steel strip enters the bath. Consequently, Fe will keep dissolving from the strip surface to the bath until an intermetallic alloy layer envelops the entire surface of the substrate. Detailed SEM and FIB-TEM characterisation was applied to the alloy layer formed at the Fe-foil/coating alloy interface. A wide range of new microstructural observations were obtained, which provide an important basis for understanding the evolution of the alloy layer structure, assisted by thermodynamic simulations. In particular, the experimental results identified that lateral growth of the nucleated Fe4Al13 compound envelops the strip surface in just ~33 ms (0.033 s). Since the dissolution of Fe from the steel strip into the bath will be terminated once such an intermetallic layer has formed, this IMC lateral growth time defines the Fe dissolution time. This is the first time in this field that the Fe dissolution time from the strip surface has been experimentally identified. Mass balance calculations at different foil preheat temperatures were further performed to determine (i) the amount of total Fe dissolved, (ii) the amount of Fe consumed by the alloy layer and (iii) how much Fe has entered the coating bath to form dross particles subsequently. The results from the current research show that the strip entry temperature or preheat temperature plays a vital role in the hot-dip coating process. For the commercial process, the experimental results suggest that the strip entry temperature below the 548oC in AM coating alloy is suitable for the overall commercial process. VIII Based on the systematic experimental studies and in-depth characterisation, a physical model has been proposed to describe the six key steps that have happened at the Fe-foil or strip surface in the coating bath in the first few hundred milliseconds. The experimental results and theoretical calculations presented in this thesis provide a unique new knowledge basis for the continuous hot-dip metal coating business.