Effect of Oxygen Partial Pressure during Annealing on the Selective Oxidation and Galvanizing of a 1.5% Al TRIP-Assisted Steel E. M. Bellhouse and J. R. McDermid McMaster Steel Research Centre, McMaster University, Hamilton, Ontario, Canada Keywords: Galvanizing, TRIP-Assisted Steels, Selective Oxidation Abstract Selective oxidation during annealing prior to galvanizing and reactive wetting during galvanizing of a 1.5% Al TRIP-assisted steel was studied. Three different atmospheres were tested during annealing prior to galvanizing: -53 ºC dew point (dp) with N 2 -20% H 2 , -30 ºC dp with N 2 -5% H 2 and +5 ºC dp with N 2 -5% H 2 . The chemical composition of the steel surface as determined with X-ray photoelectron spectroscopy (XPS) showed selective oxidation of Mn, Al and Si. The oxide morphology was studied with scanning Auger microscopy (SAM) and three different oxide morphologies were identified; nodules, grain boundary oxidation and oxide films. Good reactive wetting during galvanizing was observed at -53 ºC dp and -30 ºC dp; wetting was poor at the +5 ºC dp with an increased number of bare spots in the Zn coating and a discontinuous Fe 2 Al 5 layer. Introduction In the automotive industry advanced high strength steels (AHSS) are increasingly being used to satisfy the demands for improved passenger safety and decreased vehicle weight which translates to decreased fuel consumption and automotive emissions. Among this class of AHSS are transformation induced plasticity (TRIP)-assisted steels which have both high strength and ductility. The high strength of TRIP steels allows for thinner cross sections of these steel to be used without compromising passenger safety [1, 2]. Furthermore, the large area under the stress strain curve translates to excellent energy absorption; this is further enhanced at high strain rates such as those which would occur in the event of a collision [3, 4]. To be used in automotive exposed parts, galvanizing of these steels is essential to provide corrosion protection. Galvanizing via the continuous galvanizing process is the most cost effective means of achieving this. One of the challenges which must be overcome before these steels can be successfully galvanized is that selective oxidation of the steel surface during annealing prior to galvanizing can result in poor reactive wetting, causing bare spot defects in the galvanized coating [5-7]. Another challenge in processing these materials is that the heat treatment used to produce the TRIP steel microstructure and mechanical properties is not necessarily compatible with the temperatures required for hot dip galvanizing. The heat treatment typically used to produce the TRIP steel microstructure and mechanical properties consists of intercritical annealing followed by an isothermal bainitic transformation (IBT), typically carried out at 370 – 450 ºC [8-10]. However, the zinc bath in the CGL process must be kept at approximately 460 ºC and the steel strip should be above this temperature when entering the zinc bath and this can result in partial decomposition of the high