The Effect of Continuous Galvanizing Process Parameters on the User Properties of Hot-Rolled Transformation Induced Plasticity Steels

A hot rolled C-Mn-Si TRIP steel was subjected to three different heat treatment schedules compatible with continuous galvanizing line capabilities and the effect of the heat treatment schedules on the microstructure and mechanical properties was determined. It was found that for heat treatment schedules employing relatively high cooling rates (6 - 9°C/s) from the intercritical annealing temperature (800°C) and short overaging times at the galvanizing bath temperature (455 - 470°C), the desired microstructure of ferrite, bainitic ferrite and high volume fraction of C-rich retained austenite was produced and the desired mechanical properties achieved. For the heat treatment schedule using more moderate cooling rates (3 - 4°C/s) from the intercritical annealing temperature and higher overaging temperatures (480°C), carbide precipitation took place and the resultant microstructure contained a low volume fraction of highly C-rich retained austenite and did not meet the mechanical property targets. It was also found that the transformation of retained austenite to martensite and the value and evolution of the strain hardening coefficient with applied strain was sensitive to the heat treatment schedule.