Influence of H2-N2 atmosphere composition and annealing duration on the selective surface oxidation of low-carbon steels

The selective surface oxidation of the Ti-stabilized interstitial free (Ti-IF) and dual phase (DP) steel during recrystallization annealing at 820 degrees C under the variety of exposure time and composition of protective H-2-N-2 atmosphere at low dew point (-40 degrees C) was examined. It was found that Mn, Al, Si and Cr oxides are formed in all cases, but higher Mn and Si concentration in DP steel leads to an increase in a density of coarse nonwettable MnSiO3 particles. An increase in annealing time enhances surface coverage with oxides, while increased H, content in gas atmosphere results in less coverage with external oxides improving the wettability. The influence of the stronger reducing atmosphere is more exhibit on Ti-IF steel where after 120 s of annealing in 15vol.%H-2-85vol.%N-2 atmosphere the wetting angle theta stabilized to a value of the order of 6 degrees. The presence of surface BN and the TiN particles also affects the full surface coverage by external oxides. (C) 2005 Elsevier Ltd. All rights reserved.