Selective Oxidation and Reactive Wetting of 2Mn-Si Steels During Continuous Hot-Dip Galvanizing

The influence of annealing process parameters i.e. oxygen partial pressure and annealing time, on the selective oxidation and reactive wetting of Fe-0.1C-2Mn-1.3Si (wt.%) and Fe-0.1C-2Mn-1.7Si (wt.%) advanced high strength steels during continuous galvanizing was investigated. The effect of the addition of 0.05 wt.% Sn on the selective oxidation and reactive wetting of the Fe-0.1C-2Mn-1.7Si (wt.%) steel was also determined. The experimental steels were annealed at intercritical annealing temperatures at which an intercritical microstructure of 50 ferrite -50 austenite (vol.%) was obtained, calculated for each grade of steel using THERMO-CALC software considering the local equilibrium-non partitioning (LE-NP) formalism, for annealing times of 60 to 600 s in a N2-5 vol.% H2 gas atmosphere, where the oxygen potential was controlled by varying the dew point (dp) at 223 K (–50 °C), 243 K (–30 °C) and 278 K (+5 °C). For reactive wetting studies, samples were immersed in a 0.2 wt.% dissolved Al continuous galvanizing bath. The oxide chemistry, spatial distribution and morphology were found to be affected by the annealing process parameters. As a consequence, the reactive wetting of the steels by the continuous galvanizing bath could be controlled via controlling these parameters For the Fe-0.1C-2Mn-1.3Si (wt.%) alloy, annealing under the lowest pO2 223 K (–50 °C) dp process atmosphere led to the formation of a thick, compact, multi-layer oxide comprised of film-like SiO2 and MnSiO3 with granular MnO on top. Insignificant internal oxidation was observed in this case. Increasing the process atmosphere pO2 using the 243 K (–30 °C) dew point resulted in a considerable increase in the depth of internal oxidation and decrease in the thickness of external oxides, which is indicative of the occurrence of the transition from external to internal oxidation. Under this annealing condition, surface oxides were composed of thinner film-like MnSiO3, Mn2SiO4 and SiO2 oxides, compared to the 243 K (–50 °C) dp process atmosphere, as well as MnO oxides. Further increasing the process atmosphere pO2 via raising the dew point to 278 K (+5 °C) dp resulted in modification of the surface oxides from film-like to nodule-like MnO, MnSiO3 and Mn2SiO4 particles. Prolonged annealing times were also found to increase either the thickness of external oxides or depth of internal oxidation. For the Fe-0.1C-2Mn-1.7Si (wt.%) alloy, similar modifications in the morphology of the external oxides were observed after increasing the process atmosphere dew point from 223 K (–50 °C) to 278 K (+5 °C) dp, due to the transition from external to internal oxidation. Although steel surface was covered with thick, compact film-like SiO2, MnSiO3 and MnO under the 223 K (–50 °C) dp process atmosphere, thinner and less compact film-like and plate-like SiO2, MnSiO3 oxides and MnSiO3 nodule-like particles were identified on the surface under the 243 K (–30 °C) and 278 K (+5 °C) dp process atmospheres, respectively. MnSiO3 and Mn2SiO4 oxide particles as well as network of MnSiO3 embracing SiO2 core were found, respectively, in the bulk microstructure and along the grain boundaries in the steel subsurface. Detailed investigations into the growth kinetics of the external and internal oxides revealed that both thickness of the external oxides and depth to which internal oxides formed increased parabollically with increasing annealing time. Poor reactive wetting was obtained for both alloys when using the lowest pO2 273 K (–50 °C) dp process atmosphere due to the formation of thick, compact oxide layer on the steel surface, which formed a barrier between the substrate and Zn bath, preventing the required Fe dissolution from the substrate surface for the formation of the desired  Fe2Al5ZnX interfacial phase. This is while a well-developed interfacial Fe-Al intermetallic layer, indicative of a good reactive wetting, was observed for both alloys when using the two higher pO2 process atmospheres (i.e. 243 K (–30 °C) dp and 278 K (+5 °C dp)), due to the presence of either thinner film-like or plate/nodule-like oxides on the steel surface after annealing. This external oxide morphology facilitated contact between the Zn-alloy bath and the substrate via a variety of mechanisms, including interfacial infiltration through cracks due to thermal coefficient of expansion mismatch between the external oxide and substrate as well as aluminothermic reduction, which resulted in Fe dissolution from the substrate and consequent formation of the Fe-Al intermetallics. Moreover, the addition of 0.05 wt.% Sn to the Fe-0.1C-2Mn-1.7Si (wt.%) steel significantly improved the reactive wetting by the continuous galvanizing bath, which was found to be due to its beneficial effect on reducing the oxidation growth rate by segregating to the steel surface to about 150 times greater than its bulk concentration, occupying the potential sites for oxide nucleation, therefore altering the morphology of surface oxides. Under the lowest pO2 273 K (–50 °C) dp process atmosphere, the addition of Sn reduced the compactness of the external granular MnO oxides and increased their size. Under the intermediate pO2 243 K (–30 °C) dp process atmosphere, the addition of Sn altered the morphology of the surface oxides from large, thick and compact MnSiO3 and SiO2 particles to small, thin and less compact ones. Sn had its most significant effect on external oxide alteration under the highest pO2 278 K (+5 °C) dp process atmosphere, which was exemplified by changing their morphology from film-like and fine, closely spaced nodule-like MnSiO3 particles to big, widely spaced ones. The modified oxide morphology obtained due to the addition of Sn to the steel chemistry facilitated contact between the Zn-alloy bath and the substrate, thus resulted in the formation of smaller and more compact interfacial Fe-Al crystals and lower bare areas compared to the reference steel, therefore improved the reactive wetting during continuous galvanizing.

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