Influence of Gas Atmosphere Dew Point on the Galvannealing of CMnSi TRIP Steel
Abstract
The Fe-Zn reaction occurring during the galvannealing of a Si-bearing transformation-induced plasticity (TRIP) steel was investigated by field-emission electron probe microanalysis and field-emission transmission electron microscopy. The galvannealing was simulated after hot dipping in a Zn bath containing 0.13 mass pct Al at 733 K (460 °C). The galvannealing temperature was in the range of 813 K to 843 K (540 °C to 570 °C). The kinetics and mechanism of the galvannealing reaction were strongly influenced by the gas atmosphere dew point (DP). After the galvannealing of a panel annealed in a N2+10 pct H2 gas atmosphere with low DPs [213 K and 243 K (‑60 °C and ‑30 °C)], the coating layer consisted of δ (FeZn10) and η (Zn) phase crystals. The Mn-Si compound oxides formed during intercritical annealing were present mostly at the steel/coating interface after the galvannealing. Galvannealing of a panel annealed in higher DP [263 K and 273 K, and 278 K (‑10 °C, 0 °C, and +5 °C)] gas atmospheres resulted in a coating layer consisting of δ and D (Fe3Zn10) phase crystals, and a thin layer of D1 (Fe11Zn40) phase crystals at the steel/coating interface. The Mn-Si oxides were distributed homogeneously throughout the galvannealed (GA) coating layer. When the surface oxide layer thickness on panels annealed in a high DP gas atmosphere was reduced, the Fe content at the GA coating surface increased. Annealing in a higher DP gas atmosphere improved the coating quality of the GA panels because a thinner layer of oxides was formed. A high DP atmosphere can therefore significantly contribute to the suppression of Zn-alloy coating defects on CMnSi TRIP steel processed in hot dip galvanizing lines.
- Publication:
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Metallurgical and Materials Transactions A
- Pub Date:
- November 2013
- DOI:
- 10.1007/s11661-013-1867-8
- Bibcode:
- 2013MMTA...44.5081C
- Keywords:
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- Coating Layer;
- Phase Crystal;
- Trip Steel;
- Intercritical Annealing;
- Energy Dispersive Spectroscopy Spectrum