Numerical Simulation Of Layer Growth During Galvannealing Process And Application For Optimizing Galvannealing Parameters

The demand for zinc-coated steel sheet has risen dynamically in recent years in theautomotive industry, and the galvannealing coating (GA) has received most attentionbecause of its excellent spot weldability along with superior corrosion resistance andbetter paintability. In the galvannealing process, the steel is first immersed in an alumi-num containing (approximately0.135wt.%) zinc bath and then given a post coating heattreatment. This heat treatment causes the zinc in the coating to inter-diffuse with thesubstrate iron to form several Zn-Fe intermetallics phases, including zeta (), delta (),gamma1(1), gamma (). The problem of the flake off and the powdering during stamp-ing or forming of coated steel doesn’t be solved well. Therefore, it becomes important tounderstand the optimal microstructure and the better way of galvannealing process, whichshould be the primary study to solve the problem of the powdering.In the present work, a mathematical model for predicting the growth of zeta, delta,and gamma phases during the galvannealing process has been developed by using funda-mental diffusion data based on the primary diffusion species of zinc. The galvannealingmay be characterized as a complicated finite/semi-infinite, multi-phase, volume diffusionproblem. This implies that the diffusion of zinc in the three phases,,, and Γ, is to beconsidered along with the movement of the four interphase boundaries/,/,/Γ, Γ/.The model has taken the incubation period of the Fe2Al5layer breakdown, the amount ofinitial iron contributing to the coating during the breakdown of Fe2Al5and the beginningtime of formation of Γ into consideration. The problem can be solved numerically. Acomputer program was written in C language to obtain the simulation results. The valuesof iron content in the coating the thickness of Γ and the zinc concentration-distanceprofiles were predicted and compared to the data obtained from experimental studies.With the increase of galvannealing temperature, the growth rate of the phase increases,while the growth rate of the phase decreases. At the same iron content in the coating,the thickness of the Γ phase layer is thicker at a higher temperature.According to establishing the model of the phase becomeing unstable with thegalvannealing temperature above803K in the Fe-Zn diagram, the model has achieved thesimulation of the non-isothermal glavannealing process. A calculation software of phases growth has been developed based on Visual C++6.0. It has the advantage of inputtingcomprehensive parameters, the precise results, simple operation, friendly interface andhigh reliability. It will take a significant practical application in optimizing the galvan-nealing parameters during galvannealing process.It was studied the type of galvannealing process of first high-temperature and thenlow-temperature by the means of both experiment and simulation. The conclusion of theexperiment is the same as the one of simulation. Much attention should be paid to the newtype of the galvannealing process of first high annealing temperature and then lowannealing temperature.