Effect of coating structure on powdering resistance of galvannealed steel sheet

Abstract

Effect of coating structure on powdering resistance of galvannealed steel sheet was investigated and the followings are clarified. 1) Higher galvannealing temperature deteriorates powdering resistance. But slower cooling from higher temperature (500 approximately 550°C) improves powdering resistance. 2) In the case of lower galvannealing temperature (=440°C), the powdering resistance is rather good, although the cooling rate doesn't affect powdering resistance. 3) Exfoliation of galvannealed steel sheet after tensile test correlates with amount of powdering evaluated from draw bead test. 4) Γ1-phase (Fe5Zn21) is formed along the interface of coating galvannealed at lower temperature (≤500°C) or cooled slowly from high galvannealing temperature. 5) The decrease of Γ1 ratio [Γ1/(Γ+Γ1)] deteriorates powdering resistance.