Segregation and Selective Surface Oxidation at the Intermediate Steps of Recrystallization Annealing

Abstract

High strength steels for automotive industry undergo recrystallization annealing in N2-H2 gas atmosphere prior to hot dip galvanizing. Segregation and selective surface oxidation of the alloying elements (Al, Mn, Si, Cr etc.) depending on their extend, can be a serious problem for subsequent galvanizing as the wettability of these oxides with zinc is poor. Moreover, the H2 uptake from the annealing atmosphere majorly depends on the surface evolution during recrystallization annealing. In order to understand the surface phenomena during annealing of multicomponent alloy (i.e. steel), a systematic approach on model alloys is needed. In this work, selective surface oxidation of Mn, Al and the reduction of native Fe oxides in Fe 2 wt. % Mn and Fe 3 wt. % Al binary model alloys have been investigated by interrupting the recrystallization annealing cycle at the desired temperature. The specimens were annealed to various temperatures (200-800 °C) in N2-5%H2 gas atmospheres with a dew point of 30 °C. It has been found that the segregation and selective oxidation of the alloying elements starts at 300 °C whereas the significant reduction of native Fe oxides takes place at 400 °C. Further increase of temperature, increases the surface coverage by forming the oxide islands.