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The present paper contributes in the discussion of heterogeneous gas/metal reactions by discussing the influence of the dew point (dp) during intercritical annealing on the sub-surface constitution. Annealing trials with different Advanced High Strength Steels (AHSS) were carried out and the element distribution within the sub-surface was analysed by glow discharge optical emission spectrometer (GD-OES). For modelling purpose the gained element distribution data were adjusted in a way that the selective oxidation products were considered within the sub-surface element profiles. Several transformation temperatures along the depth profiles according to the adjusted GD-OES data have been computed for the dp À30 8C and þ5 8C. In all cases the according models have been implemented in Matlab. The thermodynamic data have been obtained via the Matlab-ThermoCalc interface. Empirical equations have been applied for the determination of the bainite and martensite start temperatures. It is shown that the sub-surface constitution and the transformation temperature differ significantly. In most cases the change of the transformation temperature within the surface and sub-surface reaches up to 100 8C compared to the bulk. For a Dual-Phase (DP) Steel with C(0.15%)Mn(1.7%)Al(1.7%)Cr(0.5%), the change of the A e1 -temperature is with 300 8C significantly higher. However one has to keep in mind that the 2-phase composition can not be assumed to be constant during intercritical annealing. It is therefore concluded that for the purpose of simulating the selective oxidation processes during intercritical annealing of AHSS the continuous change of the sub-surface constitution must be incorporated in the future work.

In this study the influence of Mn on galvannealed coatings of 1.7% Mn‐1.5% Al TRIP‐ and 23% Mn X‐IP®‐steels was investigated. It is shown that the external selective oxides like Mn, Al and Si of the TRIP steel which occur after annealing at 800 °C for 60 s at a dew point (DP) of ‐25 °C (5% H2) hamper the Fe/Zn‐reaction during subsequent galvannealing. Preoxidation was beneficially utilized to increase the surface‐reactivity of the TRIP steel under the same dew point conditions. The influence of Mn on the steel alloy was investigated by using a 23% Mn containing X‐IP®‐steel which was bright annealed at 1100 °C for 60 s at DP ‐50 °C (5% H2) to obtain a mainly oxide free surface prior to hot dip galvanizing (hdg) and subsequent galvannealing. As well known from the literature Mn alloyed to the liquid zinc melt stabilizes δ‐phase at lower temperatures by participating in the Fe‐Zn‐phase reactions, it was expected that the metallic Mn of the X‐IP®‐steel increases the Fe/Zn‐reactivity in the same manner. The approximation of the effective diffusion coefficient (Deff(Fe)) during galvannealing was found to be higher than compared to a low alloyed steel reference. Contrary to the expectation no increased Fe/Zn‐reaction was found by microscopic investigations. Residual η‐ and ζ‐phase fractions prove a hampered Fe/Zn‐reaction. As explanation for the observed hampered Fe/Zn‐reaction the lower Fe‐content of the high‐Mn‐alloyed X‐IP®‐steel was suggested as the dominating factor for galvannealing.