Selective surface oxidation of 590 MPa TRIP steel and its effect on hot-dip galvanizability

In order to gain better understanding of the selective surface oxidation and its influence on the galvanizability of a transformation-induced plasticity (TRIP) assisted steel containing 1.5 wt.% Si and 1.6 wt.% Mn, a model experiment has been carried out by depositing Si and Mn (each with a nominal thickness of 10 nm) in either monolayers or bilayers on a low-alloy interstitial-free (IF) steel sheet. After intercritical annealing at 800 °C in a N₂ ambient with a dew point of-40^°C, the surface scale formed on 590 MPa TRIP steel exhibited a microstructure similar to that of the scale formed on the Mn/Si bilayer-coated IF steel, consisting of Mn₂SiO ₄ particles embedded in an amorphous SiO₂ film. The present study results indicated that, during the intercritical annealing process of 590 MPa TRIP steel, surface segregation of Si occurs first to form an amorphous SiO₂film, which in turn accelerates the out-diffusion of Mn to form more stable Mn-Si oxide particles on the steel surface. During hot-dip galvanizing, particulate Fe₃O₄, MnO, and Si-Mn oxides were reduced more readily by Al in a Zn bath than the amorphous SiO₂ film. Therefore, in order to improve the galvanizability of 590 TRIP steel, it is most desirable to minimize the surface segregation of Si during the intercritical annealing process.