Effect of dew point on the formation of surface oxides of twinning-induced plasticity steel

Yunkyum Kim, Joonho Lee, Kwang Soo Shin, Sun Ho Jeon, Kwang Geun Chin

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Abstract

The surface oxides of twinning-induced plasticity (TWIP) steel annealed at 800 C for 43 s were investigated using transmission electron microscopy. During the annealing process, the oxygen potential was controlled by adjusting the dew point in a 15%H2-N2 gas atmosphere. It was found that the type of surface oxides formed and the thickness of the oxide layer were determined by the dew point. In a gas mixture with a dew point of - 20 C, a MnO layer with a thickness of ~ 100 nm was formed uniformly on the steel surface. Under the MnO layer, a MnAl2O4 layer with a thickness of ~ 15 nm was formed with small Mn2SiO4 particles that measured ~ 70 nm in diameter. Approximately 500 nm below the MnAl 2O4 layer, Al2O3 was formed at the grain boundaries. On the other hand, in a gas mixture with a dew point of - 40 C, a MnAl2O4 layer with a thickness of ~ 5 nm was formed on most parts of the surface. On some parts of the surface, Mn 2SiO4 particles were formed irregularly up to a thickness of ~ 50 nm. Approximately 200 nm below the MnAl2O4 layer, Al2O3 was found at the grain boundaries. Thermodynamic calculations were performed to explain the experimental results. The calculations showed that when aO2 > ~ 1.26 × 10 - 28, MnO, MnAl2O4, and Mn2SiO 4 can be formed together, and the major oxide is MnO. When a O2 is in the range of 1.26 × 10- 28-2.51 × 10- 31, MnO is not stable but MnAl2O4 is the major oxide. When aO2 < ~ 2.51 × 10- 31, only Al2O3 is stable. Consequently, the effective activity of oxygen is considered the dominant factor in determining the type and shape of surface oxides of TWIP steel.

Original languageEnglish
Pages (from-to)138-145
Number of pages8
JournalMaterials Characterization
Volume89
DOIs
Publication statusPublished - 2014 Mar 1

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Keywords

  • Annealing
  • Continuous annealing line
  • Dew point
  • Surface oxide
  • Twinning-induced plasticity steel

ASJC Scopus subject areas

  • Mechanical Engineering
  • Mechanics of Materials
  • Materials Science(all)
  • Condensed Matter Physics

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