Research On The Powdering Mechanisms Of The Zinc Coating System On Galvannealed(GA) Sheet Steel

For its high corrosion resistance, excellent formability, weldability and paintabilityperformance, the galvannealed (GA) steel sheet produced by continuous hot-dip zinccoating on steel sheet is widely used in automotive industry. However, the powderingdefect of zinc coating on galvannealed (GA) sheet steel usually occurs during stampingforming. From the influence factors, such as microstructure, residual stress, interfacestrength and the stress state of coating, the powdering mechanisms of the zinc coatingsystem on galvannealed (GA) sheet steel has been studied. First, the H220BD galvanized(GI) sheet steel provided by Shanghai Bao Steel Co. Ltd. were galvannealed usingdifferent parameters. Then the residual stress of the galvannealed zinc coating and theinterface strength between zinc coating and substrate steel were measured choosingappropriate methods, and the stress states of zinc coating during the stamping forming ofGA sheet steel were calculated and simulated by FEM using these measured results.Finally, the optimal galvannealing parameters were confirmed by checking the powderingmass lose and comparing the properties of the galvannealed zinc coatings.First, the H220BD galvanized sheet steels were galvannealed in a salt bath furnaceunder450℃,500℃,550℃and holding5s,10s,30s,120s for each temperature to obtain12kinds of zinc-coated GA steel sheets. It is found that the main component of the zinccoatings on all GA steel sheets is the δ phase, and the thickness of δ phase increases withgalvannealing time, and large amount of δ phase may appear on the surface of coating aftergalvannealing an enough long time. It is also found that the powdering mass lose of thezinc coating galvannealed under500℃is less then those obtained under450℃or550℃.The powdering mass lose of the galvannealed zinc coating first increases then slightlyreduces with the prolongation of the galvannealing time, and the powdering defectbecomes more serious under a higher galvannealing temperature holding for a samerelatively long time (>30s).The residual stress of the zinc coating on galvannealed (GA) steel sheets, obtainedusing different galvannealing parameters, was measured by nano-indentation. It is foundthat a110Mpa-240Mpa tensile residual stress exists in the zinc coating galvannealed for arelatively long time (>5s), and the residual stress first increases then decreases with theprolongation of galvannealing time, but generally reduces as the galvannealingtemperature increases. The residual stress and the powdering mass lose of the zinc coatings galvannealed under500℃and550℃both holding10s and120s are small. On thecontrary, the residual stress and the powdering mass lose of those coatings galvannealedunder450℃and550℃both holding30s are larger.A novel universal blister test method was developed to measure the interface strengthof coating/substrate material system, and the interface strength between galvannealed zinccoating and steel substrate obtained using this new method is3.8995J/m2-174.888J/m2. Itis found that the interface strength decreases rapidly first with the prolongation ofgalvannealing time, after a certain time, then slightly increases with galvannealing time butkeeps stabilized finally. During a short galvannealing time, the interface strength decreaseswith the increase of temperature, after galvannealing a certain long time, the effect oftemperature on the interface strength bacomes little. Moreover, the interface strength of thezinc coatings galvannealed under450℃and550℃both holding30s is small but theirpowdering mass lose is large. On the contrary, the interface strength of those coatingsgalvannealed under500℃is larger (>9J/m2) and their powdering is slighter.After checking the powdering mass lose and comparing the interface strength andresidual stress, the conclusions can be obtained as: Either the δ phase is too thin or toothick, the powdering of the galvannealed zinc coating is serious. The larger the residualstress or the lower the interface strength, the powdering mass lose is greater. The optimalparameters are galvannealing under500°C holding about120s.The above research work can provide theoretical basis to solve the powderingproblem of the zinc coating on GA steel sheet, and contributes to improve the quality ofour GA steel plate and to promote the development of the continuous galvanizing industry.