Selective Oxidation of TWIP Steel During Continuous Annealing Lawrence Cho, and Bruno C. De Cooman  The selective oxidation of Al-free and Al-added TWIP steel after full austenitic annealing at 8008C in a N 2 þ 10%H 2 gas atmosphere with a dew point of 178C was investigated by means of transmission electron microscopy. A thick MnO layer was formed at the surface of Al-free TWIP steel after the recrystallization annealing. Small crystalline c-xMnO  SiO 2 (x > 2) particles and amorphous a-xMnO  SiO 2 (x < 0.9) particles were found at the MnO/ steel interface. In the subsurface, the Mn depletion resulted in the formation of a narrow ferrite layer. The annealing of the Al-added TWIP steel also resulted in the formation of a thick MnO surface layer. At the MnO/steel interface, Kirkendall voids were formed between the amorphous a-xMnO  SiO 2 (x < 0.9) oxide and crystalline c-xMnO  Al 2 O 3 oxide in the case of Al-added TWIP steel. In the subsurface, a thin layer was depleted of Mn and the original austenite had transformed into ferrite. Internal oxidation of Al to Al 2 O 3 and the formation of crystalline c-xMnO  Al 2 O 3 (x > 1) compound oxide particles were found to occur at the grain boundaries of the Mn-depleted ferritic zone. The present contribution highlights the implications of the selective oxidation of TWIP steels for their processing in continuous annealing and continuous hot dip galvanizing lines. 1. Introduction Innovations in steel for automotive applications are driven by the need to achieve the reduction of vehicle mass, the lowering of greenhouse gas emissions, the drastic increase of gas mileage, and the improvement of passenger safety. These requirments explain the current trend in automotive materials development toward ultra-high strengths for structural reinforcement, superior ductility for ease of pre- ss forming, and large energy absorption capability for vehicle crashworthiness. This motivates the search for new strain-hardening mechanisms which increase the strength of steel without loss of ductility. Strain-induced phase transformations, mechanical twinning, and shear band-induced plasticity are known to enhance substan- tially both strength and ductility of materials. Steels which undergo twinning induced plasticity (TWIP) have extra- ordinary mechanical properties which make them attrac- tive for automotive applications. [1,2] The processing of TWIP steels in continuous hot dip galvanizing (HDG) lines for applications in automotive body-in-white construction is essential because thin gauge TWIP steel sheet must be resistant to perforation corrosion. Figure 1 illustrates schematically the various stages in a continuous galvanizing line (CGL) where a surface modi- fication process takes place. The gas atmosphere of the industrial annealing furnaces in continuous HDG lines leads to the reduction of the iron oxides formed during cold rolling. Mn, Si, and Al are, however, subject to selec- tive oxidation. The presence of film forming surface oxides, in particular, the amorphous a-xMnO  SiO 2 (x < 0.9) and a-SiO 2 oxides, leads to the deterioration of the wettability of the steel by molten Zn and prevents the formation of the Fe 2 Al 5x Zn x inhibition layer. [3–5] The selective oxidation at the surface and in the subsurface of IF, DP, and TRIP steels is well documented in several publications. [5–11] The selec- tive oxidation of TWIP steels, which have Mn contents in the range of 15–25 mass%, has however not yet been reported, and the morphology, size, distribution, and com- position of the selective oxides which may form on TWIP steel during continuous annealing or prior to HDG is not known. The Al added in the Zn bath leads to the formation of the Fe 2 Al 5x Zn x inhibition layer. This layer is formed on the steel surface in the initial stages of the hot dipping process. [  ] L. Cho, Prof. B. C. De Cooman Materials Design Laboratory, Graduate Institute of Ferrous Technology, Pohang University of Science and Technology, Pohang, South Korea Email: decooman@postech.ac.kr DOI: 10.1002/srin.201100296 www.steel-research.de ß 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim steel research int. 83 (2012) No. 4 391