EFFECT OF MICROSTRUCTURE OF BASE STEEL ON FE-ZN ALLOY GROWTH DURING GALVANIZING OF AN INTERSTITIAL-FREE STEEL

The effect of the microstructure of the base steel on the formation an d growth behavior of intermetallics on interstitial free steel during galvanizing under various processing conditions (substrate entry tempe rature, Al content in galvanizing bath) was investigated from the view point of the surface morphology of the intermetallics. In addition, th e surface morphology of galvannealed coating was also examined. The de pendency of Fe-Zn intermetallics growth behavior on the crystal orient ation of the base steel was observed clearly in galvanizing in 0.10 ma ss% or less Al bath, especially at low entry temperatures. In general, zeta crystals are precipitated orderly on (111)alpha, whereas those a re precipitated disorderly on (001)alpha and (10l)alpha at the outset of the formation of Fe-Zn intermetallics, and the growth rate of Fe-Zn intermetallics on (001)alpha and (101)alpha is larger than that on (1 11)alpha. As an exception, the formation of Fe-Zn intermetallics on (0 01)alpha and (101)alpha at the outset of Fe-Zn interaction is suppress ed at the entry temperature of 673 K in 0.10 mass%. Al bath, although the clump of orderly precipitated zeta is formed on (111)alpha. The gr owth of Fe-Zn intermetallics during galvannealing also depends on crys tallographic orientation of the alpha Fe grain of the base steel, bein g retarded on (111)alpha. A large alpha Fe grain with (111)alpha on th e surface is thought to cause a concave portion on the galvannealed su rface. Consequently, it is thought that the formation and growth behav ior of Fe-Zn intermetallics on IF steel is affected not only by the al pha Fe grain boundary but also by the crystallographic orientation of the alpha Fe grain of the base steel.