The Impact of Selective Oxidation on the Phase Transformation in the Sub‐Surface of Advanced High Strength Steels

Abstract

AbstractThe 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 −30 °C and +5 °C. 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 °C 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 Ae1‐temperature is with 300 °C 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.