Effect of Heat Treatment on Formability of Hot-dip Galvanized Low Carbon Steel Sheet

Abstract

In this research, the effects of annealing heat treatment on the coating microstructure and formability of hot dip galvanized sheet were evaluated. For this purpose, samples of galvanized sheet with identical production parameters, mechanical properties, and coating thicknesses (weight) were heat treated at a temperature range of 500–540°C for 10–180 s. After heat treatment, the samples were cooled in water and dried quickly. Scanning electron microscopy (SEM) was employed before and after heat treatment to evaluate the microstructure and coating layers. Energy dispersive spectroscopy (EDS) microanalyser and x-ray diffractometer were also used to determine each layer's chemical composition and phase, respectively. Formability of the samples was evaluated using forming limit diagrams (FLDs). FLDs and strain distribution were evaluated experimentally by stretch-forming sheet samples over a hemispherical punch. The fractography of the stretched samples was accomplished using SEM. Experimental results show that surface appearance quality changes with temperature and time of heat treatment process from silvery and bright to gray and dull. Conclusions drawn from formability evaluations indicate that the growth of iron-zinc brittle intermetallic layers and complete removal of η phase in heat treatment decrease the level of forming limit diagrams. Also, heat treatment at higher temperatures and for shorter process times improves coating formability behavior.