Effects of Steel Chemistry on Surface Enrichment and Galvanizability of Automotive Steel Sheets

摘要

Effects of substrate chemistry like Mn, B, Ti, Nb, Al and S in interstitial free(IF) steel on surface enrichment and galvanizability of automotive exposed steel sheet have been investigated by optical microscopy(OM), scanning electron microscopy(SEM) and transmission electron microscopy(TEM) combined with focused ion beam(FIB) technique. It is known that Mn, Ti, Nb and Al deteriorate the surface quality of galvannealing(GA) coating because of their oxide enrichment during re-crystallization annealing. Compared to Nb added steel substrate, in Ti and Ti+Al added steel substrate, higher density of concave and/or scratch are observed. Small amount of B and S additions also drastically influences the surface characteristic of the GA coatings. Combining addition of Mn and B in steel substrate is detrimental to the quality of the GA coating by increasing the amount of surface enrichment after annealing process; from this view point the amount of B is limited below 10×10~(-6) B in automotive exposed panels. Finally, it is known that small amount of S addition between 18×10~(-6) and 150×10~(-6) decreases the surface enrichment and improves GA coating morphology.