Selective Oxidation and Reactive Wetting during Galvanizing of a CMnAl TRIP-Assisted Steel

Abstract

A transformation induced plasticity (TRIP)-assisted steel with 0.2 pct C, 1.5 pct Mn, and 1.5 pct Al was successfully galvanized using a thermal cycle previously shown to produce an excellent combination of strength and ductility. The steel surface chemistry and oxide morphology were determined as a function of process atmosphere oxygen partial pressure. For the 220 K (-53°C) dew point (dp) + 20 pct H 2 atmosphere, the oxide morphology was a mixture of films and nodules. For the 243 K (-30°C) dp + 5 pct H 2 atmosphere, nodules of MnO were found primarily at grain boundaries. For the 278 K (+5°C) dp + 5 pct H 2 atmosphere, nodules of metallic Fe were found on the surface as a result of alloy element internal oxidation. The steel surface chemistry and oxide morphology were then related to the reactive wetting behavior during continuous hot dip galvanizing. Good wetting was obtained using the two lower oxygen partial pressure process atmospheres [220 K dp and 243 K dp (-53°C dp and -30°C dp)]. An increase in the number of bare spots was observed when using the higher oxygen partial pressure process atmosphere (+5°C dp) due to the increased thickness of localized oxide films.

Stability of Retained Austenite in High-Al, Low-Si TRIP-Assisted Steels Processed via Continuous Galvanizing Heat Treatments

“…[13][14][15][16][17] Samples comprised 120 mm 9 200 mm sheets such that the rolling direction (RD) was parallel to the longitudinal axis of the sample. A uniform temperature area of 180 mm 9 90 mm is centered on the sample panel and only material from this area was analyzed.…”

“…Previous work by the research group of the present authors and others has shown that high quality galvanized coatings can be obtained for TRIP-assisted steels in which Si was either partially or completed replaced by Al. [5][6][7][8][9][10][11][12][13][14][15][16] In particular, the present authors have successfully coated two high Al, low Si (0.20C-1.5Mn-1.0Al-0.5Si and 0.20C-1.5Mn-1.5Al (wt pct)) steels [14][15][16] for which very good mechanical properties were also obtained. [17] A further challenge of processing TRIP-assisted steels within the CGL concerns the thermal cycles commonly used.…”

Microstructural evolution and oxidation behavior of hot-dip 55 wt.% Al–Zn–Si coated steels

“…One of the methods was the application of inorganic coatings [5][6][7][8] such as Alxwt.%Si (x = 0~31.9) coatings [9][10][11][12], Zinc silicate coatings [13] and zinc coatings [14,15].…”

Prediction of Oxide Phases Formed upon Internal Oxidation of Advanced High-Strength Steels

“…Mn, Cr, Si, Al) in AHSS have a higher affinity to oxygen than iron, leading to the formation of oxides of these elements during the annealing process prior to galvanizing. The presence of these alloying element oxides at the steel surface reduces the zinc wettability [3] and thus impairs the galvanizing quality [4]. Therefore, it is imperative to understand the oxidation behaviour of AHSS in N 2 plus H 2 gas mixtures with various dew points and to find conditions to mitigate the formation of external oxides after annealing.…”