Effect of the presence of alloying elements in interstitial-free and low-carbon steels on their surface composition afte
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6/8/04
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Effect of the Presence of Alloying Elements in Interstitial-Free and Low-Carbon Steels on Their Surface Composition after Annealing in Reducing Atmospheres (Dew Point 30 °C) S. FELIU, Jr. and M.L. PÉREZ-REVENGA The X-ray photoelectron spectroscopy (XPS) method is used to study the outer surfaces of interstitialfree (IF) and low-carbon (LC) steels with different alloying element contents (P, Ti, Nb, and Mn) after annealing at temperatures of 850 °C and 705 °C, respectively, for 40 seconds in reducing atmospheres (dew point 30 °C). The work discussed seeks to establish possible relationships between the bulk composition of the IF and LC steels and the contents of segregated alloying element observed by XPS on the surface of the annealed steels, as well as to establish the influence of the presence of a thin iron oxide film on the steel surface on the segregation and oxidation of the alloying elements. Despite the low Mn and Si bulk steel contents and the shortness of the annealing cycle, considerable enrichment of these elements on the surface is seen, mainly as manganese and silicon oxides. The formation of a MnO layer on the annealed steel surface seems to be related to the reduction of iron oxides and the increase in the metallic Fe content. Despite its low content in LC steels, carbon also seems to diffuse towards the annealed steel surface to reduce iron oxides.
I. INTRODUCTION
THE cold rolling of steel needs an annealing treatment to improve its ductility with a view to subsequent drawing and forming processes. The presence of hydrogen together with nitrogen in the formulation of the annealing atmosphere usually reduces the slight oxidation that takes place on the steel surface during rolling, but often fails to prevent the segregation of alloying elements or more active impurities present in the bulk steel and the possibility of precipitation in oxide form during annealing.[1–12] Numerous studies have considered the segregation of Si[2,8–10,13–17] and P[1–3,18–29] during the annealing process and the effect that the presence of these elements on the steel surface has on the subsequent growth of the zinc coating in hot-dip galvanizing and galvannealing processes.[12,30–33] Other elements such as Mn have received less attention, as is noted by Mintz.[1] In relation to this element, it has been recommended[34] not to exceed bulk steel contents of 1.5 pct in order to avoid problems in the galvanizing process. However, as has been noted by Olejford et al.,[11] even Mn contents of 0.5 pct can give rise to the formation of MnO islands on the surface of IF steels during annealing at 850 °C in an atmosphere with a dew point between 20 °C and 30 °C. The technological interest[35] of IF steels (Ti, TiNb, and rephosphorized) and LC steels (rephosphorized) with Mn percentages of between 0.1 and 0.5 wt pct have encouraged us to use them in this study of the effect of alloying elements on the composition of the steel surface. Many studies have been published[2,7,13,14,16,18–21,28,29,36] in
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