Thermochemical surface treatment of iron–silicon and iron–manganese alloys

Abstract

Two binary alloys (Fe-0.5 wt.% Si and Fe-1.5 wt.% Mn) and a high silicon steel were used to investigate the thermochemical phenomena occurring at the sample free surface during simulation annealing cycles aimed to achieve a hot-dip galvanizing-compatible surface condition.The samples were annealed for 60 s at 800-810• C in atmospheres composed of nitrogen and hydrogen (either 0 or 5 vol.% H 2 ), the dew points of which were varied from −60 to 0 • C, corresponding to water vapour contents ranging from 10 to 6000 ppm. The surface chemical conditions were characterized by in situ x-ray photoelectron spectroscopy (XPS). Annealing in a nitrogen-hydrogen atmosphere was shown to lead silicon and manganese to diffuse to the surface and become oxidized, the amount of external selective oxidation decreasing with increasing atmosphere vapour content. After annealing in pure and dry nitrogen, the thin native iron oxide films are reduced to metallic iron, while free surface silicon or manganese oxides are definitely less abundant than after annealing in N 2 -H 2 gas mixtures. Although iron oxide is thermodynamically stable in water-vapour-containing nitrogen atmospheres, our results suggest that, provided the dew point is low enough, the oxygen present in the thin native iron oxide film is transferred to more stable oxide forms (silicon or manganese oxides). The transition from iron oxide reduction to iron oxidation takes place at some critical water vapour content threshold that is dependent on the steel composition. The phenomena observed in this study are explained in terms of the balance between the oxygen inward flow and the solute alloying element outward flow. In this context, several possible gas-metal and exchange reactions are discussed.

Investigation into the surface selective oxidation of dual‐phase steels by XPS, SAM and SIMS

“…A number of studies carried out on iron-silicon and iron-manganese alloys have shown clearly that increasing the water vapour content drastically decreases external selective oxidation. 10 This is because the thus reinforced oxygen inward flow into the steel substrate traps the alloying elements in internal oxide particles.…”

“…3 This is usually easier said than done, owing to external selective oxidation problems. 4 It is indeed well-established today that classical annealing atmospheres promote alloying element segregation to the surface, depending on the heat cycle and the steel chemistry. As far as the latter is concerned, the most important parameters relate to the nature of the elements involved, their contents, diffusion coefficients and affinity with other elements inside and surrounding the specimen.…”

“…4 The aim of our study was to determine the influence of the water vapour content on the quantity of external and selective oxidation and the different oxide morphologies present at the free surface. For this purpose, a tight preparation chamber was connected directly to the XPS facility described elsewhere.…”

Selective Oxidation during the Austenitic Annealing of a CMnSi Steel

“…The binding energy of the Mn2p 3/2 electrons is about 641.8eV and 642.7eV for annealing at 950°C and 850°C respectively. The Si2p peaks have 2 constituents at 102.7eV and 104.5eV, respectively corresponding to Mn 2 SiO 4 and SiO 2 [7]. After annealing at 850°C, the main peak corresponds to SiO 2 while at 950°C he corresponds to Mn 2 SiO 4 .…”

“…It is proved that selective surface oxidation during recrystallization annealing considerably hampers the galvanisability [1,2,3,4]. Surface selective oxidation of a Si and Mn alloyed high strength steel has already been studied in various conditions [5] but not yet thoroughly during austenitic annealing. Previously it was shown by Mahieu [6] that there is less external selective oxidation on a CMnSi steel composition provided that the temperature is high enough to ensure a fully austenitic annealing.…”