The effect of Sn microalloying on the selective oxidation kinetics and reactive wetting of advanced high strength steels

Abstract

Mn-containing Advanced High Strength Steels (AHSSs) are promising candidates for automotive applications, but continuous galvanizing for corrosion protection by Zn-based coatings can be challenging, as external Mn oxides that form during annealing under the resident N2-H2-H2O process atmosphere are detrimental to reactive wetting. Microalloying with 0.01 and 0.03 at.% Sn was evaluated as strategy to decrease selective oxidation kinetics and enhance reactive wetting of Fe-(6-10)Mn (at.%) alloys. During annealing, Sn segregated to grain boundaries, metal/oxide interfaces, and the steel surface. The majority of Sn surface coverage was established during linear heating, where the high defect density in the as-received cold-rolled substrate accelerated Sn diffusion. At metal/surface oxide interfaces, Sn segregation was lower than at the free surface and decreased with increasing Mn concentration due to modified external oxide morphology and an increase in internal interfacial area reducing Sn availability for continued segregation. In the Fe-6Mn alloy, 0.01 and 0.03 at.% Sn were equally effective in decreasing selective oxidation due to Sn enrichment at the surface reducing O availability by blocking adsorption sites and lowering O solubility, as well as due to reduced transport kinetics of O and Mn along grain boundaries and metal/oxide interfaces in the Sn-added alloys. The decreased external oxide thickness resulted in lower fractions of uncoated surface and increased Fe2Al5Znx formation at the substrate/coating interface, indicating increased reactive wetting. For the Fe-10Mn alloy, however, the Sn micro-additions did not significantly alter either external oxidation and coating quality, as effective Sn segregation was lowered and increased oxide coverage reduced the effectiveness of segregated Sn in decreasing selective oxidation kinetics. Upon dipping, Sn segregation is removed from the external surface, potentially by dissolution into the galvanizing bath, thereby reducing the amount of Sn in the final steel product. Overall, Sn micro-additions were shown to be a promising strategy to facilitate continuous galvanizing of AHSSs containing up to 6 at.% Mn.