Selective Oxidation of TWIP Steel During Continuous Annealing |
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Authors: | Lawrence Cho Bruno C De Cooman |
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Affiliation: | 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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Keywords: | TWIP steel selective oxidation dew point phase transformation |
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