Coating Formability Evaluation Method for Hot-Dip Galvannealed Sheet 920175

Hot-dip galvanneal sheet is produced throughout the world and several companies have developed specific testing methods to evaluate the adhesion properties of the coating to the steel sheet. There is no universal standards for individual companies to compare the quality of their coating to other commercial products. This paper reports on a light absorption technique to assess the galvanneal coating powdering characteristics after bend testing. Specimens are taped and subjected to a bend test, and then the tape is stripped, placed on a glass slide and evaluated in a spectrophotometer. The spectrophotometer measures the amount of light absorbed through the tape and powder. Galvanneal coating formability as it was affected by several processing variables were determined including: steel substrate, bath aluminum levels and galvannealing times and temperatures. It was found that die interaction played a significant role in coating formability. Approximately 80% of powdering was caused by die interaction. Also, galvanneal produced from different substrates and processing parameters with a range of coating iron contents performed similarly during powdering tests. The results show that substrate material and processing parameters have a secondary effect on powdering properties. The coating microstructure, which was constant for all five materials, defines the powdering properties of galvanneal Zn coatings.