Investigation of performance limitations in continuous hot-dip galvanizing units associated with product defects

The goals of the work were to search and systematize the speed limits of the steel strip during hot-dip galvanizing, associated with the threat of product defects. Since speed can be combined with many other factors, this paper provides an overview of the known causes of common defects. The causes were grouped taking into account the operations of individual sections of continuous hot-dip galvanizing units. To determine the circumstances in which defects occur, a method was used that involved step-by-step stratification of retrospective data and comparison of the distribution density of influencing factors for defective and non-defective products. The method was applied in the analysis of the data on defects at the MMK Metallurgy plant in Turkey, obtained in 2020–2021. Twenty-one types of acceptable and unacceptable defects that occur during galvanizing of DX51D steel were selected for analysis. Twenty-two technological parameters were taken as factors, including the strip speed. For each selected type of the defects, a set of influencing factors is determined, and for some types of defects, the alleged causes of their occurrence are indicated. It is shown that the relationship observed for many types of defects with the strip speed can actually be caused by other factors. We determined the types of defects, the probability of which increases with an increase in the speed or the level of its change. The paper proposes measures aimed at preventing the increase in the proportion of defective products along with the performance gain.