5th International High Manganese Steel Conference - HMNS 2022

This paper will present the effects of substrate chemistry and process atmosphere pO2 on the selective oxidation and reactive wetting of a series of prototype 0.2C-6Mn-xSi-yAl-zCr third-generation advanced high strength steels (3G AHSS). The substrates were first austenitized and quenched to room temperature, flash pickled, intercritically annealed and galvanized, where all annealing treatments were conducted in a N2-5% H2 process atmosphere under a variety of controlled dew points. All substrates demonstrated 3G-compatible properties.

Austenitizing the panels at a dew point of -30°C or -10°C resulted in the selective oxidation of Mn, Si, Al, and Cr, resulting in a compact external oxide layer as well as an extensive internal oxide network in a near pure Fe matrix. The external oxides were highly soluble in the hydrochloric acid pickling solution, and the resulting intermediate surfaces were relatively oxide-free apart from some discrete, dispersed nano-oxides. The ensuing intercritical annealing at a dew point of -30°C, -10°C, or +5°C did not significantly alter the surface structures.

High-quality and adherent galvanized coatings were formed after a 4 s immersion in a conventional 0.2 wt% Al (dissolved) bath. The substrate/coating interfacial structures were investigated using transmission electron microscopy equipped with electron energy loss spectroscopy (TEM-EELS), and it was determined that the primary reactive wetting mechanism was direct wetting of the substrate Fe by the galvanizing bath. Secondary mechanisms included oxide wetting, oxide cracking and lift-off, oxide bridging by the liquid bath metal, and infiltration of the bath metal into the substrate through channels left by the selective dissolution of some internal oxides during flash pickling. A fully-developed Fe2Al5-xZnx interfacial layer was observed. These results show that the two-stage processing route is promising for the production of galvanized medium-Mn 3G AHSS.