| Galvannealed steel sheet(hereinafter called GA steel sheet) has been more and more used in automotive industry with its excellent corrosion resistance, coating properties, weldability and advantages of high surface strength. The main existing defect is the powdering and exfoliation of GA steel sheet which is the problem researchers are trying to solve. The amount of powdering and exfoliation in stamping process not only has a direct impact on stamping performance, but also deteriorate its corrosion resistance and easily lead to surface defects affecting the appearance after painting. Therefore, it's a technology development difficult for galvannealed steel sheet production to produce coating with good anti-powdering performance while meeting other performance.In this paper, GA steel sheets in varying alloying degrees were made by alloying galvanized steel sheets, the stretching and bending deformation were in-situ observed in vacuum chamber of SEM, and the coating powdering process of GA steel sheet during the forming process was studied. Using OM,SEM,XRD and electrolytic stripping to study the buildup and structure of the GA steel sheet coating, and the anti-powdering performance of the GA steel sheet coating was researched using double punch test as well. On the basis of those, it was proposed that the powdering mechanism of GA steel sheet coating in varying alloying degrees during stretching and bending deformation.The results showed that cracks in the insufficient alloying GA steel sheet coating increased not too much while the steel sheet was bending. Cracks generated on the coating/steel substrate, and extended along the coating/steel substrate interface, so the coating tended to peel off easily. Standard alloying GA steel sheet and excessive alloying GA steel sheet coating cracks generated fromδphase, then extended perpendicular to the coating/substrate interface toГ/steel substrate increased much more obviously. As for badly excessive alloying GA steel sheet, cracks generated from/phase, then extended perpendicular to the coating/substrate interface throughг1phase and finally to the coating surface, increased obviously on the tensile side.Pre-cracks were existed in the coating of GA steel sheet before deformation. When bending pre-cracks on the stretched side extended, new cracks was generated and extended when deformation continues. While coating on the compressed side suffered a dislocation slip, a part of coating upheaved. As bending continues, compressed coating of insufficient alloying GA steel sheet had no significant change; Dislocation slip occurred in compressed coating of standard alloying GA steel sheet and excessive alloying GA steel sheet coatings at the pre-cracks, and the slip became severe as iron content increased. And for badly excessive alloying GA steel sheet, new cracks were generated on theг1/гinterface and extended along theг1/гinterface, and propagated to the coating surface by complex type.
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