The Effect of Bi on the Selective Oxide Formation on CMnSi TRIP Steel

Abstract

The formation of an external oxide layer of Mn, Si, and Al during the continuous annealing of advanced high strength steel (AHSS) usually results in the deterioration of the coating quality of the galvanized AHSS. In this study, the effect of Bi addition on the selective oxidation and the galvanizablity of CMnSi transformation-induced plasticity (TRIP) steels was investigated by hot dip galvanizing laboratory simulations. Bi-added TRIP steels were intercritically annealed at 820 oC in N2 + 5%H2 gas atmosphere with -60 oC dew point and galvanized in a 0.22 wt.% Al containing Zn bath at 460 oC for 4s. The oxide morphology and the galvanizability of the reference and three kinds of Bi-added TRIP steel were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and 3D atom probe tomography(APT). Bi formed a Bi-enriched surface layer during the intercritical annealing. A decrease of the oxygen permeability was observed with increasing Bi addition. The internal oxidation was suppressed in Bi-added CMnSi TRIP steel. The surface oxide morphology was changed from a continuous layer morphology to a more lens-shaped morphology. The galvanizability of the Bi-added TRIP steel was significantly improved. Fine and dense Fe2Al5-xZnx crystals were formed at the steel/coating interface of Bi-added TRIP steel, whereas coarse and discontinuous Fe2Al5-xZnx crystals were formed at the steel/coating interface of reference TRIP steel. Bi-enriched surface layer was not observed after hot dip galvanizing. The mechanism for the improvement of the galvanizability of a CMnSi TRIP steel by Bi additions seemed to the combination effects of the change of the oxide morphology and the dissolution of the Bi enriched surface layer during immersion of the strip in the Zn bath.