Effect of annealing temperature on the surface and subsurface microstructure of Al-added TWIP steel

Annealing experiments were carried out in a N-2-5 vol% H-2 atmosphere with a dew point of +10 degrees C to investigate the effect of annealing temperatures ranging from 600 degrees C to 850 degrees C on the surface and subsurface microstructure of Al-added TWIP steel. The depth profiles of Fe, Al, Mn, O and C in the annealed samples along a 3 mu m depth from the surface were measured by GD-OES. High-resolution cross sections were prepared by FIB and characterized by TEM. It was found that the annealing temperature plays an important role in the surface and subsurface formation of Al-added TWIP steel. Due to the abundant formation of external MnO and decarburization, the content of Mn and C in the subsurface area decreases to a critical low level and results in the formation of a continuous ferrite layer. In addition, the type, shape, size and distribution of the internal oxides in the subsurface area are greatly affected by the annealing temperature. When the annealing temperature increases from 700 degrees C to 800 degrees C, the film-like internal MnAl2O4 along the grain boundaries grows coarser, and some large internal MnO is formed. Furthermore, a layer of protruded ferrite grains without any internal oxides inside the grains or along the grain boundaries appears just beneath the external MnO layer, the amount of which increases with the annealing temperature.