Effect of B on Surface Oxidation Behavior and Phosphatability of Si–Mn-added Cold-Rolled Steel Sheets

The effect of B on the surface oxidation behavior and phosphatability of cold-rolled steel sheets was investigated using 0.001 wt% B-added and B-free steels containing 0.6 wt% Si and 2.0 wt% Mn. The specimens were annealed at 800 °C in a 5 vol% H2-N2 atmosphere with a dew point of -50 °C. The surface oxides of the annealed samples were analyzed by GD-OES, FT-IR, SEM-EDX and TEM. The annealed steel sheets were then subjected to zinc phosphate treatment, and the effect of the surface oxides on phosphatability was evaluated by SEM-EDX. In the initial stage of annealing, fine granular Mn2SiO4 mainly formed and film-like SiO2 partly formed on both steels. As the soaking time at 800 °C increased, the granular Mn2SiO4 increased in the B-free steel. In contrast, in the B-added steel, the granular Mn2SiO4 coarsened, MnSiO3, MnO and B2O3 formed, and the film-like SiO2 formation area expanded. Addition of B reduced the melting point, causing coarsening of Mn2SiO4, exposing the base steel. This results in a difference in the oxygen potential between the exposed area of the steel and the oxide covered area. This local inhomogeneity of the oxygen potential changes the surface oxide species of the B-added steel. To elucidate the reason for the poor phosphatability of the B-added steel, a SEM-EDS analysis of the steel surface in the initial stage of zinc phosphate treatment was conducted, revealing that the coarse Si-Mn complex oxides and large film-like SiO2 inhibited the zinc phosphate reaction.