Effect Of Surface Condition Of Steel On Microstructure Of Hot-dip Galvanized Coating

In the process of hot-dip galvanizing,the rust removal is a very important pretreatment step.In the traditional hot-dip galvanizing process,the method of pickling and removing rust was usually adopted.However,over pickling and environmental pollution almost caused by waste acid.Therefore,it was of great significance to remove rust mechanically before hot-dip galvanizing instead of pickling.In this paper,the microstructure and properties of the steel after shot-blasting were analyzed,as well as the influence of Fe-Zn atomic diffusion and microstructure of the hot-dip galvanizing coating in the subsequent hot galvanizing process.The structure of hot-dip galvanized coating were divided into four layers.The coating consisted of the ? phase,the ? phase,the ? phase and the ? phase from the surface to the inside,respectively.The coating thickness was uniform after pickling treatment.The Fe-Zn‘‘outbursts' ' were formed at the raised areas of steel surface after shot-blasting,and the thickness of the ? phase of here was very thin.In addition,the thickness of each phase in the flat plating layer was thinner than that in the pickling treatment samples.There was a positive correlation between the change of coating thickness and the change trend of roughness of steel matrix after shot-blasting.The lower the surface roughness,the thicker the coating layer.The thickness of coating decreased first and then increased with the shot-blasting time prolonging.Plastic deformation layer of a certain depth would be generated near the surface layer of the steel was caused by shot-blasting treatment.The grains near this area were deformed and fibrous.Many crystal defects such as dislocations and grain boundaries were produced.The nucleation of more alloy phases was promoted by the crystal defects near the surface of the steel.A large number of crystal defects such as grain boundary and dislocation density could also be used as a fast track of atomic diffusion,thus the Fe-Zn reaction was promoted.The large residual compressive stress produced by shot-blasting in the surface layer.T he surface roughness and hardness could be greatly improved by shot-blasting.The change of the thickness and microstructure of the hot-dip galvanizing was caused by the combination of these factors.