Effect of alloying elements (Ti, Nb, Mn and P) and the water vapour content in the annealing atmosphere on the surface composition of interstitial-free steels at the galvanising temperature

Abstract

X-ray photoelectron spectroscopy (XPS) analysis is used to study the outer surface of interstitial-free (IF) steels with different alloying element contents (Ti, Nb, P and Mn) after annealing and subsequent cooling to a temperature of 500°C (close to the molten zinc bath entry temperature) in atmospheres with different water vapour contents (dew point, DP=−45, −10 and 10°C). XPS measurements reflect a clear influence of the DP of the atmosphere on the oxide and hydroxide contents on the steel surface. An appreciable increase in the O/Fe atomic ratio is observed as the DP of the atmosphere rises. In general, O/Fe ratios are seen to be higher for the IF–Ti steel than for the IF–TiNb steel. Since the annealing conditions (temperature, time and DP of the atmosphere) are similar for both steels, this higher value may be related with the additional presence of titanium oxides on the IF–Ti steel surface. In the case of the IF–TiNbP steel, despite the short annealing time, significant Mn and P contents are observed on its outer surface. The considerable P increase on the steel surface is a rather unexpected fact considering the tendency of this element to be localised at the grain boundaries. With this steel the manganese content is greater on the surfaces annealed at low DPs (−45 and −10°C). In contrast, the phosphorus content seems to rise with the DP.