Influence of annealing conditions on the galvanizability and galvannealing properties of TiNb interstitial-free steels, strengthened with phosphorous and manganese

A high-strength interstitial-free (HS IF) steel, solution strengthened with P and Mn, was hot-dip galvanized and galvannealed in a laboratory hot-dip simulator. The dew point of the N-2-H-2 annealing atmosphere during recrystallization was varied, and the effect of the dew-point changes on the galvannealing kinetics was studied. Annealing in a high-dew-point N-2-H-2 atmosphere improved the wettability by the zinc and the galvannealing reactivity of the Mn-P-added IF steels considerably. Scanning electron microscopy (SEM) analysis and glow discharge optical emission spectroscopy (GDOES) depth profiles of the surface of the steel sheet and the subsurface region revealed clear differences in oxidation mode after recrystallization annealing. Plane-view SEM and transmission electron microscopy (TEM) analyses were done on the inhibition layer of the as-galvanized samples. Differences in the Mn and P contents were found. By analyzing the Mn and P contents in the coatings during galvannealing, a mechanism was proposed to explain the higher galvannealing reactivity which is observed when the dew point during recrystallization annealing is increased.