Selective Oxidation of TWIP Steel During Continuous Annealing

Authors

  • Lawrence Cho,

    1. Materials Design Laboratory, Graduate Institute of Ferrous Technology, Pohang University of Science and Technology, Pohang, South Korea
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  • Bruno C. De Cooman

    Corresponding author
    1. Materials Design Laboratory, Graduate Institute of Ferrous Technology, Pohang University of Science and Technology, Pohang, South Korea
    • Materials Design Laboratory, Graduate Institute of Ferrous Technology, Pohang University of Science and Technology, Pohang, South Korea.
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Abstract

The selective oxidation of Al-free and Al-added TWIP steel after full austenitic annealing at 800°C in a N2 + 10%H2 gas atmosphere with a dew point of −17°C 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 · SiO2 (x > 2) particles and amorphous a-xMnO · SiO2 (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 · SiO2 (x < 0.9) oxide and crystalline c-xMnO · Al2O3 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 Al2O3 and the formation of crystalline c-xMnO · Al2O3 (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.

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